Development and Present Status of Organic Superconductors

Abstract

based on the two main requirements for the conductivity, namely, (1) a uniform segregated stacking of the same kind of component molecules, and (2) the fractional CT state (uniform partial CT) of the molecules. Since TTF•TCNQ has a low-dimensional segregated stacking, it showed a metal-insulator (MI) transition (Peierls transition) below about 60 K. For TTF•TCNQ, the Peierls transition occurs by the nesting of the one-dimensional Fermi surface causing lattice distortion associated with the strong electron-phonon interaction and forms charge density wave (CDW). There are also several one-dimensional organic metals which show MI transitions by the formation of spin density wave (SDW) when the periodicity of the SDW coincides with the nesting vector of Fermi surface and no lattice distortion occurs in this case. An increase in the electronic dimensionality is inevitable to prevent the nesting of Fermi surfaces and develop superconductors. Several attempts have been made through "pressure", "heavy atom substitution", or "peripheral addition of alkylchalcogen groups" (Fig. 1). The latter two correspond to the enhancement of the self-assembling ability of the molecules. Appropriate examples taking TTF derivatives are shown in Fig. 1. Based on TMTSF molecules several superconductors under pressure have been prepared with warped one-dimensional Fermi surface since 1980 (a in Fig. 1) [9-14]. In general, the ratio of transfer energies (t// / t ⊥) is larger than 3 for one-dimensional Fermi surface and a closed two-dimensional Fermi surface is formed when t// ≤ 3t ⊥ , where t// and t ⊥ are the transfer energies along the directions of the largest and second largest intermolecular interactions. The BO (BEDO-TTF) molecules afforded stable two-dimensional metals having two-dimensional Fermi surface (b in Fig. 1) owing to the strong self-assembling ability by intermolecular S•••S and hydrogen-bonds [15], and only two superconductors are known since 1990 (Tc ≤ 1.5 K). The substitution of an ethylenedioxy group with an ethylenedithio group (BO → ET (BEDT-TTF)) destabilized the metallic state of BO compounds and provided unstable two-dimensional conductors (c in Fig. 1). Consequently, variety of superconductors and other functional solids have been developed based on twodimensional metals of ET since 1982 (Tc ≤ 13.4 K) [16-20] and its analogues (Tc ≤ 10 K) [21]. Figure 1. Strategy for chemical modification of the TTF molecule to increase (arrows) or decrease (dotted arrow) the electronic dimensionality by the aid of enhancement or suppression of the selfassembling ability of the molecules, respectively [16]. Typical Fermi surfaces of TMTSF (a: (TMTSF)2NbF6), BO (b: (BO)2.4I3), and ET (c: β-(ET)2I3) CT solids are depicted.

Understanding the Structure and Electronic Properties of Molecular Crystals Under Pressure: Application of Dispersion Corrected DFT to Oligoacenes

“…In the past, pressure has been used to reduce the resistivity of these semiconductors, suggesting that oligoacences could be the first single component organic molecular crystals (OMC) to show “metallic” behavior . The idea that oligoacenes (particularly pentacene) under pressure are conductors is perpetuated in recent articles and reviews. , However, a high pressure optical study of pentacene up to ∼32 GPa showed no metallization due to condensation reactions, though a steady narrowing of the HOMO–LUMO band gap was observed until the molecular identity of the acene components was compromised.…”

New Charge Transfer Cocrystals of F2TCNQ with Polycyclic Aromatic Hydrocarbons: Acceptor–Acceptor Interactions and Their Contribution to Supramolecular Arrangement and Charge Transfer

“…The very fact of cocrystal formation largely depends on the match between the HOMO and LUMO energy levels of the donor and acceptor molecules and the geometry fit of coformers, in particular their size and planarity. The electronic properties of cocrystals depend on coformers; however, the supramolecular arrangement of the latter in single crystals is just as important, as it determines the overlap of their molecular orbitals, transfer integrals, and transport network. − The most illustrative examples involve different properties of CT complexes, containing tetrathiafulvalene derivatives as donors and tetracyanoquinodimethane (TCNQ) derivatives as acceptors, which possess a different stacking mode: the polymorphs with segregated stacks have metallic conductivity, while the other modifications with alternating donor and acceptor components are insulators or semiconductors. − However, even smaller structural differences in polymorphic cocrystals with alternating stacks, though having similar HOMO and LUMO energies, may involve a significant variation of charge-transfer and transport properties …”